/*
author : abushbandit
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimize("Ofast,unroll-loops")
using namespace __gnu_pbds;
using namespace std;
#define int long long
#define pb push_back
const int N = 1e5 + 1,INF = 1e18;
void solve(){
int n,m;
cin >> n >> m;
string s;
cin >> s;
map<string,int> dis;
string h = s;
sort(h.begin(),h.end());
dis[s] = 1;
queue<string> q;
q.push(s);
int ans = INF;
n = s.size();
while(!q.empty()){
s = q.front();
q.pop();
int cnt = 0;
vector<int> vis(n);
bool check = 1;
for(int i = n - 1;i >= 0;i--){
if(vis[i]) continue;
if(s[i] == 'A'){
check = 0;
break;
}
cnt++;
int u = 1;
for(int j = i - 1;j >= 0;j--){
if(vis[j])continue;
if(s[j] == 'A'){
break;
}
u++;
vis[j] = 1;
}
for(int j = i - 1;j >= 0;j--){
if(vis[j]){
continue;
}
if(s[j] == 'A'){
u--;
vis[j] = 1;
}
if(u == 0){
break;
}
}
if(u != 0){
check = 0;
break;
}
}
if(m == cnt && check){
ans = min(ans,dis[s] - 1);
}
h = s;
for(int i = 0;i < n - 1;i++){
swap(s[i],s[i + 1]);
if(dis[s] > dis[h] + 1 || dis[s] == 0){
dis[s] = dis[h] + 1;
q.push(s);
}
swap(s[i],s[i + 1]);
}
}
cout << ans << "\n";
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
int t = 1;
// cin >> t;
while(t--){
solve();
}
}
